This invention relates to the processing of copper-nickel-tin alloys to achieve optimum mechanical strengths for given levels of ductility, and to the resulting products.
While the highest mechanical strengths are usually associated with steel alloys, the combination of good mechanical strength, ductility, electrical conductivity and corrosion resistance exhibited by the copper alloys make them favored candidates for a wide variety of applications for which higher strengths would otherwise be desirable. Among the copper alloys, the beryllium-coppers have up to the present time exhibited the highest mechanical strengths, which have been achieved by the mechanism known as precipitation hardening. Such hardening, however, is normally accompanied by a substantial loss in ductility. For example, the highest 0.01 yield strengths (yield strength is a measure of resistance of a material to permanent deformation, a property which is particularly significant in the specification of materials for springs, relay elements, wire connectors or other similar flexible articles) which have been reported for such alloys (containing about 2 weight percent beryllium) range from about 170,000 to 175,000 pounds per square incn for textured sheet or strip. However, such strengths are accompanied by ductilities of the order of about 5 percent (ductility being defined herein as the reduction in cross-sectional area of a specimen tested in tension to its point of failure), too low for most applications requiring forming operations after hardening. Overaging to recover needed ductility is accompanied by a drop in 0.01 yield strength. For example, the 2 percent beryllium alloy may exhibit a 0.01 yield strength of 110,000 to 120,000 psi for a ductility of about 50 percent reduction in area. This drop in 0.01 yield strength as well as the high raw materials cost of beryllium and the expense of special handling due to its toxicity may make other copper alloys more desirable for certain applications.
The trend toward miniaturization and the need for increased reliability of mechanical components, particularly in the communications field, have been major factors contributing to a growing demand for alloy materials having higher yield strengths in combination with good to excellent ductilities, corrosion resistance and conductivities than have heretofore been available and at costs which would make them competitive with existing alloys. Representative of recent progress in meeting such demand is U.S. Pat. No. 3,663,311 issued to G. Y. Chin and R. R. Hart on May 16, 1972 and assigned to the present assignee. This patent describes processing of copper-beryllium, cupro-nickel, nickel-silver and phosphor-bronze alloys to achieve optimum yield strengths for given levels of ductility. Such progress invites the investigation of other alloy systems.
One such alloy system, the copper-nickel-tins, exemplified by the 5 weight percent nickel, 5 weight percent tin alloys, in general would be expected to have better corrosion resistance, better solderabilities and conductivities comparable to those of the copper-beryllium alloys. However, while good hardening response to cold working of these alloys has been observed, it has been accompanied by severe embrittlement rendering the material useless for most commercial applications. See, for example, E. M. Wise and J. T. Eash, Metals Technology, Jan. 1934, No. 523, page 238. Thus, with the exception of some use as age hardenable casting alloys prior to 1950, these alloys have not found significant widespread commercial use.
The discussion below is in terms of a compositional range in which the claimed alloys represent an economical alternative to cooper-beryllium alloys; specifically, this preferred range is from 4 to 40 weight percent nickel and from 3 to 12 weight percent tin, remainder copper. However, the claimed alloys may find application where cooper-beryllium is not customarily utilized. For example, alloys containing smaller amounts of nickel and/or tin are of practical interest in the manufacture of articles such as relay elements where they can be used as substitutes for the phosphor bronze alloys in current use. Specifically, alloys containing as little as 2 percent nickel and as little as 2.5 percent tin are of commercial interest.